This invention relates generally to optical systems. More specifically, it relates to a novel method and apparatus for servo-based spectral array alignment. The present invention can be used to construct a variety of servo-based optical devices including spectral power monitors, multiplexers and demultiplexers, and optical add-drop multiplexers, which are well suited for WDM optical networking applications.
As all-optical communication networks become increasingly pervasive, a challenge to optical networking equipment makers is to provide optical components and subsystems that are robust, versatile, and cost-effective.
Contemporary optical communication networks commonly employ wavelength division multiplexing (WDM), for it allows multiple information (or data) channels to be simultaneously transmitted on an optical fiber by using different optical wavelengths, thereby significantly enhancing the information bandwidth of the fiber. The prevalence of WDM imposes a particular need for a line of optical systems that are capable of separating a multi-wavelength optical signal into a spatial array of spectral channels according to wavelength, so that these spectral channels can be separately detected by an array of optical power sensors, as in the case of spectral monitors; directed into an array of input/output ports (e.g., optical fibers), as in the case of optical multiplexers/demultiplexers; or dynamically routed by an array of micromirrors according to a predetermined scheme. In such optical systems, it is essential that the requisite alignment between the spectral channels and the designated beam-receiving devices (i.e., optical power sensors or micromirrors) be maintained over the course of operation, and robust with respect to environmental effects such as thermal and mechanical disturbances.
Conventional optical devices in the art, however, typically employ precision alignment, which dictates stringent fabrication tolerances and painstaking alignment during assembly. Moreover, there are no provisions provided for maintaining the requisite alignment over the course of operation; and no mechanisms implemented for overcoming shift in the alignment due to environmental effects such as thermal and mechanical disturbances. Altogether, these shortcomings render the prior optical devices characteristically high in cost, cumbersome in size and operation, and prone to degradation in performance.
In view of the foregoing, it is desirable and would be a significant advance in the art to provide for a new line of optical devices in which the optical alignment is actively controlled during operation in a simple, robust, and cost-effective construction.
The present invention provides a method and apparatus for servo-based spectral array alignment in optical systems. The optical apparatus of the present invention comprises an input port, for providing a multi-wavelength optical signal along with a reference signal; a wavelength-disperser for spatially separating the multi-wavelength optical signal and the reference signal by wavelength into multiple spectral channels and a reference spectral component in a spectral array with a predetermined relative arrangement; a beam-receiving array including a reference-wavelength-sensing element and a plurality of beam-receiving elements, positioned such to receive corresponding ones of the reference spectral component and the spectral channels; and a servo-control unit for maintaining the reference spectral component at a predetermined location on the reference-wavelength-sensing element and thereby ensuring a particular alignment between the spectral channels and the beam-receiving elements.
In the present invention, a xe2x80x9cspectral channelxe2x80x9d is characterized by a distinct center wavelength and associated bandwidth, and may carry a unique information signal as in WDM optical networking applications. A xe2x80x9creference signalxe2x80x9d (and the corresponding xe2x80x9creference spectral componentxe2x80x9d) generally refers to any optical signal characterized by a well-defined (and stable) center wavelength that does not substantially overlap with any of the wavelengths of the spectral channels under consideration. Further, the terms xe2x80x9creference signalxe2x80x9d (or xe2x80x9creference spectral componentxe2x80x9d) and xe2x80x9ccalibration signalxe2x80x9d (or xe2x80x9ccalibration spectral componentxe2x80x9d) may be used interchangeably in this specification.
A beam-receiving element in the present invention should be construed broadly as embodying any optical element that corresponds with at least one spectral channel. By way of example, a beam-receiving element may be an optical power sensor, an optical fiber, a micromirror, a focusing lens, or an optical modulator. The beam-receiving elements may be configured to be in a one-to-one correspondence with the spectral channels. The beam-receiving elements may also be configured such that a subset of beam-receiving elements each corresponds with a plurality of the spectral channels.
In the aforementioned optical apparatus of the present invention, the input port may be provided by a fiber collimator coupled to an input optical fiber. In the event that the multi-wavelength optical signal is transmitted by the input optical fiber and the reference signal is provided by a reference light source, an optical combiner (e.g., a fiber-optic fused coupler) may be used to couple the reference light source to the input optical fiber. This provides a simple way of coupling both the multi-wavelength optical signal and the reference signal into the same input port. Alternatively, a particular optical signal (e.g., a service channel in an optical network) may be designated to serve as the reference signal on a network level, as might be in WDM optical networking applications, and transmitted along with various WDM signals through the communication system. The wavelength-disperser may be provided by a diffraction grating, such as a ruled diffraction grating, a holographic diffraction grating, an echelle grating, a transmission grating, a dispersing prism, or other types of wavelength-separating means known in the art. The reference-wavelength-sensing element may be a position sensitive detector, a split detector, or a quadrant detector known in the art, each allowing the impinging position of an optical beam to be monitored by way of the output electric signals produced by the sensing element. The optical apparatus of the present invention may further include a beam-focuser, e.g., one or more focusing lenses, for focusing the spectral channels along with the reference spectral component into corresponding focused spots impinging onto the respective beam-receiving elements.
The employment of a reference signal along with the corresponding reference-wavelength-sensing element allows the present invention to advantageously exploit a servo-control unit for maintaining the requisite alignment between the spectral channels and the beam-receiving elements. The servo-control unit may be in the form of a processing element working in conjunction with an alignment-adjusting element capable of adjusting the alignment of the spectral channels along with the reference spectral component. The processing element serves to monitor the real-time impinging position of the reference spectral component onto the reference-wavelength-sensing element by processing the output signals received from the reference-wavelength-sensing element, and to provide feedback (or servo) control of the alignment-adjusting element accordingly, so as to maintain the reference spectral component at the predetermined location and thereby ensure the requisite alignment between the spectral channels and the respective beam-receiving elements. The alignment-adjusting element may be an appropriate actuation device coupled to the beam-receiving array, for causing the reference-wavelength-sensing element and the beam-receiving elements to move in tandem and thereby adjusting a relative alignment between the spectral array and the underlying beam-receiving array. The alignment-adjusting element may alternatively be a beam-steering device, such as a dynamically adjustable mirror in optical communication with the input port and the wavelength-disperser, for adjusting the alignment of the input multi-wavelength optical signal along with the reference signal. The alignment-adjusting element may also be an actuation device coupled to the wavelength-disperser (e.g., a diffraction grating), for causing the wavelength-disperser to move (e.g., rotate) and thereby adjusting the alignment of the spectral channels along with the reference spectral component. In the event that a focusing lens is employed as a beam-focuser in an optical apparatus of the present invention, the alignment-adjusting element may also be in the form of an appropriate actuation device coupled to the focusing lens, for controlling the impinging positions of the spectral channels along with the reference spectral component onto the beam-receiving array.
Moreover, the optical apparatus of the present invention may employ one or more auxiliary reference signals, along with corresponding auxiliary-reference-wavelength-sensing elements, to complement the aforementioned function of the reference spectral component. Accordingly, the servo-control unit may advantageously make use of a combination of the alignment-adjustment methods as described above to actively control the position as well the pitch of the spectral array, thereby ensuring a more robust alignment between the spectral channels and the respective beam-receiving elements.
As such, the employment of the servo-control unit enables the optical apparatus of the present invention to actively correct for shift in alignment owing to environmental effects such as thermal and mechanical instabilities over the course of operation, and therefore be more robust in performance. An additional benefit of using such a servo-control unit is manifested in relaxed fabrication tolerances and precision during initial assembly, rendering the optical apparatus of the present invention a more adaptable and cost-effective construction.
The present invention further provides a method of performing spectral alignment of a multi-wavelength optical signal. The inventive method entails combining a multi-wavelength optical signal with a reference signal; spatially separating the multi-wavelength optical signal and the reference signal by wavelength into multiple spectral channels and a reference spectral component having a predetermined relative arrangement; impinging the reference spectral component at a predetermined location, such that the spectral channels impinge onto designated locations in accordance with the predetermined relative arrangement; and maintaining the reference spectral component at the predetermined location by way of servo-control, thereby ensuring that the spectral channels stay aligned at the designated locations.
In the aforementioned method of the present invention, the servo-control mechanism may be accomplished by monitoring the real-time impinging position of the reference spectral component and adjusting the alignment of the reference spectral component along with the spectral channels accordingly, so as to maintain the impinging position of the reference spectral component at the predetermined location and the spectral channels at the respective designated locations.
The method of the present invention may further include the step of focusing the spectral channels along with the reference spectral component into corresponding focused spots. It may additionally include the step of optically detecting the spectral channels at the designated locations, so as to provide a power spectrum of the detected spectral channels; the step of re-directing the spectral channels, so as to dynamically route the spectral channels according to a predetermined scheme; or modulating one or more characteristics of the spectral channels.
As such, a new line of servo-based optical systems, including spectral power monitors and optical multiplexers/demultiplexers, can be constructed according to the present invention, to meet the ever-challenging demands of optical networking applications.
The novel features of this invention, as well as the invention itself, will be best understood from the following drawings and detailed description.